In tubular heat exchangers, it is necessary to provide the limiting guide wall around the curved portion of the U-shaped tubes. Heretofore, the limiting guide walls have been constructed, for instance, as sheet-metal wings which conform to the arcuate outer contour of the curved portions of the tubes. The limiting wall is integral with or secured as a separate element to the housing. The temperature and, consequently, the expansion of the wall differs from that of the tube assembly. Therefore, in order to permit free displaceability of the tube assembly, a gap or spacing is required between the guide wall and the adjacent curved portions of the tubes of the assembly.
Such a spacing results in a relatively large leakage flow for the fluid which is external of the heat exchanger tubes, namely the heated gas.
As a result of this construction, two important disadvantages result which reduce the effectiveness of the heat exchanger:
First, this quantity of hot leakage gas does not participate in the heat exchange process and, second, it "shoots" , at the outlet of the gap, at a relatively high velocity into the normal region of discharge of the heated gas downstream of the tube assembly to produce turbulence in this downstream region and, thus, also relatively intense non-uniformity in flow which, together with the first disadvantage, leads to a relatively large reduction in the degree of heat exchange.
U.S. Pat. No. 3,746,083 discloses a heat exchanger of the above type in which the limiting guide wall conforms to the curved portion of the tubes in spaced relation therewith and is a fixed part of the housing which receives the tube assembly and guides the hot gases. The guide wall directly rests against the curved portions of the tubes of the assembly through the intermediary of pressure elements which bridge the gap between the limiting guide wall and the curved portions of the tubes. Thereby, the leakage flow gap is substantially sealed off, with the consequence, however, of a non-definable heat-exchange process in the region of the curved portions of the outer tubes of the assembly. Namely, a flow of hot gases guided uniformly around the curved portions and the outer tubes of the assembly is not effected. In particular, thermally produced differential expansions between the tubes and the housing and/or the outer limiting guide wall which are produced particularly, in the curved region of the tubes are not taken into consideration in this construction.